Phototropic glass

ABSTRACT

A transparent and durable phototropic glass comprising a borosilicate glass body containing therein microcrystals of silver halide, copper oxide and lanthanum oxide, and process for making the phototropic glass which comprises melting a batch for the said glass body, forming and cooling the molten glass, heat treating said glass to cause the silver halide to crystallize, and cooling the glass.

United States Patent [191 Murakami et al.

[ Oct. 16, 1973 PHOTOTROPIC GLASS Inventors: Yoshio Murakami, Itami; Makoto Kume, Amagasaki, both of Japan Assignee: Nippon Sheet Glass Co., Ltd.,

Osaka, Japan Filed: Jan. 25, 1972 Appl No.: 220,715

Related US. Application Data Continuation-in-part of Ser. No. 821,978, May 5, 1969, abandoned.

Foreign Application Priority Data May 17, 1968 Japan 43/33331 July 10, 1968 Japan 43/48699 U.S. Cl 106/54, 106/53, lO6/DIG. 6 Int. Cl. C03c 3/26, C03c 3/14, C030 3/08 Field of Search l06/DIG. 6, 52, 53,

References Cited UNITED STATES PATENTS 12/1970 Suzuki et a1. I06/DIG. 6

3,197,296 7/1965 Eppler et al. I06/DIG. 6 3,208,860 9/1965 Armistead et al. 106/DIG. 6 2,150,694 3/1939 Morey...= 106/47 Q 3,653,933 4/1972 Tsunekawa 106/54 OTHER PUBLICATIONS Volf, M. 8., Technical Glasses, London, 1961, pages 88,132, 268, 411.

Primary Examiner--Helen M. McCarthy Att0rneyE. F. Wenderoth et al.

2 Claims, No Drawings PHOTOTROPIC GLASS This application is a continuation-in-part of Serial No. 821,978, filed May 5, 1969, now abandoned.

This invention relates to a new phototropic glass and in particular to a transparent and durable phototropic glass which is promptly darkened by exposure to ultraviolet radiation or visible radiation of short wave-length but promptly reverts to its original state immediately after the radiation ceases.

Various phototropic glasses having a small amount of microcrystalline silver halides dispersed in their body have been known heretofore. Most of these are those whose body comprises silicate glass and, in generaL' they do not respond quickly to changes in light intensity, i.e. do not color and decolor immediately. A phototropic glass whose body comprises borate glass has been recently developed to obtain prompt coloration and decoloration. Such a phototropic glass comprising a borate glass body is disclosed in U.S. application Ser. No. 606,530 now Pat. No. 3,653,833. While this phototropic glass possesses superior properties with respect to its transparency and promptness of coloration and decoloration, it has the shortcoming that its durability is not quite satisfactory. When the durability of the glass is low, certain of its constituents result in the impairment of its transparency.

It has now been found that a phototropic glass containing borate glass in its body, i.e a phototropic glass comprising a borosilicate glass body, is improved remarkably in its durability by the incorporation in the glass body of a specified amount of lanthanum oxide.

It is therefore an object of the present invention to provide a phototropic glass comprising a borosilicate glass body which possesses prompt coloration and decoloration properties as well as improved durability. Another object is to provide a process for making such a glass.

In one aspect the invention comprises a phototropic glass comprising a glass body having an analysis of- 30-80 mole B 0.5-50 mole SiO the total amount of 8,0 plus SiO being in the range of 60 85 mole percent, 5 20 mole M 0 1 12 mole La,o 5 25 mole percent of at least one metallic oxide selected from BaO, SrO, PbO, ZnO and CaO,

wherein the total amount of BaO and SrO combined is at least one-half of the total mole of the metallic oxide, 0 mole percent of at least one alkali metal oxide, 0 3 mole TiO and 0 2 mole ZrO the glass also comprising at least 0.005 percent by weight of the glass body of copper oxide (reckoned as CuO) and at least 0.05 percent by weight of the glass body of microcrystals of at least one silver halide selected from AgBr and AgCl (reckoned as Ag).

As hereinabove indicated, the glass body according to the present invention is made of the principal constituents of B 0 A1 0 I .a O and R0 (where R0 stands for BaO, SrO, PbO, ZnO or CaO) to which is added SiO, and, optionally, alkali metal oxides such as Li O, Na O or K 0, TiO and ZrO The addition of a small amount of SiO is effective in stabilizing the glass, i.e. in inhibiting the information of haze in the glass. This effect is particularly pronounced when the Al,O content exceeds 16 mole percent. An amount of the SiO up to 50 mole percent is usable. On the other hand, the amount of 8,0 used is in the range of 30 80 mole percent. However, the total amount of B 0 plus SiO must be held within the range of 60 mole percent. This means that when the amount of B 0 is greater than 30 mole percent but less than 60 mole percent, SiO is present at the same time and the sum total of the two constituents is in the range of 60' 85 mole percent. When the foregoing range is deviated from, problems such as either non-vitrification, clouding of glass or uneven coloration upon exposure of the resulting phototropic glass occur.

As concerns the Al O constituent, when this is below 5 mole percent, not only does the forming of the glass become difficult but also the structure of the glass tends to become non-uniform. On the other hand, when the Al O exceeds 20 mole percent, there is the drawback that insoluble portions tend to form in the glass.

The L3 0, constituent plays an important role in improving the durability of the phototropic glass. When the amount of this constituent is less than one mole percent, the intended improvement in durability is not fully achieved, whereas when the content of La o exceeds l2 mole percent, the photosensitivity of the glass is depressed.

The amount of the RO constituent ranges 5 25 mole percent. At less than 5 mole percent, vitrification does not take place fully and insoluble portions are formed. On the other hand, when the 25 mole percent limit is exceeded, formation of a homogeneous glass becomes difficult. in addition, not only is the molding operation rendered difficult but also the uneveness of the darkening of the glass upon exposure to light is aggravated.

A further requirement in the case of the RO constituent is that at least one-half of its total mole percent must be accounted forby either BaO or SrO or a mixture of these. When these conditions are not satisfied, not only does haze tend to form in the glass during its after heat treatment, but also a satisfactory photosensitivity cannot be obtained.

The effect of improvement in the durability of the glass by means of the La o is further enhanced in the case where Pb O is present as the RO constituent. Therefore, PhD is preferably present in the glass. Further, since a large amount of Cao tends to depress the photosensitivity of the glass, it is preferable, although not required, that the CaO be held to below two-fifths of the total mole percent of the RO constituent.

Although the alkali metal oxides need not necessarily be added, they have the effect of making the melting of the glass composing materials easier as well as lowering the heat treatment temperature after the formation of the glass. Up to 10 mole percent of these alkali metal oxides can be added. The presence in the glass of an amount exceeding the foregoing amount has adverse effects on the durability of the glass.

As other constituents, up to 3 mole percent of TiO and up to 2 mole percent of ZrO can be incorporated. These constituents cooperate with the foregoing La o in improving the durability of the glass. However, when the content of TiO exceeds 3 mole percent, the transparency of the glass suffers, whereas when the amount of ZrO exceeds 2 percent the photosensitivity of the glass is depressed.

' The phototropic glass of this invention contains dispersed in the glass body comprising the various aforesaid constituents, a minute quantity of microcrystalline silver halide and a minute quantity of copper oxide. The microcrystals of the silver halide and copper oxide constituents appear to be mainly responsible for the phototropic property of the glass. Photosensitivity is not obtained if the silver halide alone is used, a small amount of copper oxide being essential. The silver halide may be the chloride or bromide, or both. The amount of silver halide must be at least 0.05 percent by weight, calculated as silver, based on the glass body. If it is less than this value, the density of coloration attainable is too small for the glass to be really useful. The

density of coloration attained on exposure increases nearly proportionately to the amount of silver halide present up to about 1.7 weight percent of silver halide, calculated as silver, but above this concentration the density of coloration becomes almost constant. Hence, the presence of more than about 1.7 weight percent of silver halide, calculated as silver, is not economical. The amount of the copper oxide, reckoned as CuO, is at least 0.005 percent by weight based on the glass body. An increase above 0.05 percent by weight does not demonstrate any marked improvement in the photosensitivity. Thus, no advantage is obtained by the use of more than 0.05 percent by weight of copper oxide.

The phototropic glass of the present invention described above possesses an excellent phototropic property as well as improved durability.

In another aspect the invention comprises a process for making a phototropic glass according to the invention, which comprises forming a molten batch of the borosilicate glass body of the specified composition containing the silver halide and copper oxide in the specified proportions, shaping and cooling the molten glass to solidify it, heating or holding the glass at a temperature of 550 770C. for a time at least sufficient to cause the silver halide to crystallize, and cooling the glass. 7

In the process of the invention, the starting materials for preparing the borosilicate glass body may be those used in making conventional borate glass, for example, boric acid, aluminum hydroxide, lanthanum oxide and alkaline earth metal carbonates, which are converted to their respective oxides in the melting step. To produce the silver halide, it is possible to include a nonhalide silver salt, e.g. silver nitrate, and an alkali metal halide such as sodium or potassium bromide or chloride in the charge to thus form the silver halide in the melting step. Because the loss of the alkali metal halide during melting is greater than the loss of the silver salt by volatilization, it is preferred to add a stoichiometric excess of the alkali metal halide over the silver salt. The melting conditions may be those customarily used, the starting materials being heated for l to 6 hours at a temperature of 1,l50 to 1,300C. in air or other oxidizing atmosphere. Temperature and times above or below these limits are undesirable, since when they are below the lower limits, bubbles tend to remain in the resulting molten glass, whereas when they are above the upper limits, the amounts of boric oxide and halides volatilized become large. I

The molten glass obtained in this manner is formed into a suitable shape and coated. The so obtained glass is then heat treated at 550 700C. A part of the silver halide contained in the glass is formed into very minute crystals by this heat treatment, with the consequence that photosensitivity is imparted to the glass. With heat treatment below 550C. photosensitivity is not obtained, while with heat treatment above 700C. trans-' parency of the glass is impaired. A period of time suitable for the heat treatment is from one-half to 3 hours. The heat-treated glass is then cooled.

The following example illustrates the invention.

Before presenting the example, however, the tests for the phototropic property and durability to which the samples of glass produced in the example were submitted will be described.

The phototropic property was tested by determining the optical transmittance of the glass plate before and after exposure for a period of 90 seconds to a light radiation. A' ISO-watt xenon lamp was used as the light source which was separated from the sample by a distance of about 7 centimeters. T and T, obtained as a result of the test represent respectively the transmissions to visible light of the glass plate before and after exposure to the xenon lamp radiation, expressed in percent. And hft represents the half-fading time in seconds at which the concentration of color centers after the stopping of exposure to the light radiation is at one-half that at equilibrium. More specifically for instance, in case the initial transmission (T is 90 percent and the transmission (T after 90 seconds radiation is 50 percent, the time that it takes for the transmission to recover to V T X T V X 50 67 is referred to as half-fading time (hft). This provides a measure of the rate of fading of the darkened glass or its ability to regain its original transmission. Each of these tests was conducted at room temperature on samples 3.7 mm in 'less than 10 mg/cm", including the range 0.2 6.0

mglcm thus providing an extremely durable glass. Example 1 The several starting materials in the amounts specitied in Table l were charged to a crucible which was then placed in a furnace and heated for 1 to 6 hours at 1,150 to 1,300C. in an atmosphere of air to melt the batch. Thus were produced the corresponding glass samples Nos. 1 30 indicated in Table 2 having an analysis in mole of the constituents as shown. These glasses contained in all instances, aside from the glass body constituents indicated in the table, based on the weight of said glass body, 0.5% of Ag, 0.5% of Br and 0.3% of Cl, as well as 0.01% of CuO. The crucible was then removed from the furnace, and the molten glass was flowed onto a stainless steel plate to form it into a sheet, which was then .allowed to stand and solidify. After this sheet was heat treated in a furnace under the conditions shown in Table 3, it was allowed to cool. The results of the phototropic property test of these glass samples are shown in Table 3.

The durability test was conducted on several of these samples with the results shown in Table 4. From these results it can be seen that the phototropic glass of this invention possesses excellent durability.

TABLE 1 Parts by weight Lagos AgBr NaCl C110 mixture Other SiOz ZnO

B203 A1(OH) BaCO Number 666666666666666666666666666666 2222ZZZZ2222222221212222221222 0 0 0 QQQQQQQQQQQQQQQQQQQQQQQQQQO 33 3333333333333333333333333333 333333&33&3333331333333 m33333i .5 2 &&L 21 11 CuO mixture: 99 parts 13 0; (included in quantity shown in "1320:" column) 1 part CuO.

TABLE 2 Other Z110 Lazoa PbO 2 A1203 BaO Number 4 21 .8030 0 "0 L2 Lnm TAB LE 2A Weight percent Other PhD Number 0 wm ON $0 0123156T3UU 1111232223222Z3 TABLE 3 Heat treatment T, (36) T (56) hit (sec) conditions 90 43 56 650C, 30 min. 90 54 32 675C, 30 min. 90 47 30 675C, 30 min. 90 52 30 675C, 30 min. 90 54 2S 650C, 30 min. 90 65 I6 650C, 30 min. 87 49 80 675C,30 min. 90 42 36 675C, 30 min. 88 68 2] 650C, 30 min. 90 45 60 650C, 30 min. 88 O 90 650C, 30 minv 90 59 65 650C, 30 min. 88 47 68 675C, 30 mini 90 47 57 625C, 30 min. 88 41 95 650C, 30 min. 89 62 45 650C, 30 min. 89 37 70 675C, 30 mini 88 40 43 650C, 30 min. 88 40 50 650C, 30 min. 90 52 38 675C. 30 min. 88 44 36 650C, 30 min. 90 41 40 675C, 30 min. 88 38 60 650C, 30 min. 90 34 70 650C, 30 min. 90 51 65 650C, 30 min. 90 51 27 675C, 30 min. 90 57 14 675C, 30 min. 90 53 26 650C, 30 min. 90 50 30 650C, 30 min. 87 43 45 650C, 30 min.

TABLE 4 Durability No. (mg/cm) 2 1.0 4 1.8 8 3.9 9 6.0 10 5.2 1 1 3.5

We claim:

1. A transparent and durable phototropic glass exhibiting a weight loss of less than 10 mg/cm when dipped for one hour in boiling distilled water, held for two hours at 100C in a dryer and then freed of matter adhering to its surfaces, which glass consists essentially of a glass body having an analysis of 30 mole B 0 0.5 50 mole SiO the total amount of B 0 and SiO being 60 mole percent, 5 20 mole A1 0 1 12 mole 1.350,, 5 25 mole percent of at least one metallic oxide selected from the group consisting of BaO, SrO, PbO, ZnO and CaO, wherein the total amount of BaO and SrO is at least one-half the total mole percent of the metallic oxides and 0 10 mole percent of at least one alkali metal oxide, the glass containing also at least 0.005 percent by weight of the glass body of copper oxide, calculated as 010, and at least 0.05 percent by weight of the glass body of microcrystals of at least one silver halide selected from the group consisting of AgBr and AgCl, calculated as Ag.

2. The glass according to claim 1, wherein the amount of PhD present is 0.7 5.7 mole percent and the amount of CaO present is less than 10.0 mole percent. 

2. The glass according to claim 1, wherein the amount of PbO present is 0.7 - 5.7 mole percent and the amount of CaO present is less than 10.0 mole percent. 